Owing to industrialization and urbanization, organic pollutants in water become a serious environmental killer along with the advancement of technology and industry.^[Reference Shen, Zhang, Zhou, Peng, Xie, Zhang and Pang^1–Reference He, Asi, Xiong, Shu and Li^4] Thus, it is importance to develop simple and effective methods to address these issues. A promising method for destroying a wide spectrum of organic compounds is photodegradation, which has been studied extensively for destructive oxidation of various organic pollutants. This has proved to be very effective, since many organic compounds can be degraded successfully to safer end-products such as CO₂, H₂O, and mineral acids.^[Reference Galindo, Jacques and Kalt^5–Reference Wang, Jiang, Ding, Wang, Sun, Wang and Chen^7] In particular, TiO₂ has been the focus in this field due to its unique physical and chemical properties, including high efficiency, good stability, nontoxicity, etc.^[Reference Zainal, Hui, Hussein, Abdullah and Hamadneh^8–Reference Jiang and Zeng^10] However, TiO₂ exhibits photocatalytic activity under UV light (λ < 387 nm), because its band gap is greater than 3 eV (e.g., 3.2 eV for anatase TiO₂).^[Reference Xie and Yuan^11, Reference Chen, Chen, Shi and Zhang^12] Therefore, it can utilize only about 4% of sunlight, resulting in its limited practical applications.^[Reference Kalyanasundaram and Grätzel^13–Reference Primo, Corma and García^15] Therefore, it has been a challenging problem to fabricate photocatalysts that work under visible light, which covers 43% of sunlight.

Aiming at developing visible light-driven photocatalysts, a great deal of effort has been devoted to the hierarchical structure development and band gap regulation because shape and band energy of photocatalysts play a vital role on their physical/chemical properties.^[Reference Zhao, Wei, Lu, Wang and Duan^16–Reference Li, Liu and Huang^19] As a group of V–VI–VII semiconductors, bismuth oxyhalide compounds have attracted considerable attention due to their remarkable photocatalytic activities under visible-light illumination. The structure of these bismuth oxyhalides comprises a layer of [Bi₂O₂]²⁺ slabs interleaved by double slabs of halogen atoms.^[Reference Zhang, Liu and Zhou^20] The unique catalytic properties of these materials were attributed to the high dipole moment (value above 2.00 D) within most of the BiOX crystals, the indirect band gaps, high redox potential of the holes, low electron–hole recombination rates, and the easy formation of O vacancies.^[Reference Zhang, Liu and Zhou^21]

We^[Reference Jiang, Wang, Wang, Hu, Xi, Zhou, Wang, Wang and Chen^22–Reference Jiang, Wang, Wei, Li, Xi, Hu, Tang, Wang, Wang, Wang and Chen^24] and others^[Reference Yao and Yu^25–Reference Jiang, Yang, Yang, Kong, Jones, Xiao and Edwards^27] have recently reported a new family of composite photocatalysts based on bismuth oxyhalides that synthesized via a hydrothermal method. These composite photocatalysts exhibited a unique photocatalytic activity under visible light in degradation of Rhodamine B (RhB) and other organic pollutants in water. In this communication, we introduced a facile solvothermal route to prepare the Mn-doped bismuth oxide bromide (BiOBr) microspheres with unique hierarchical 3D nanostructure which showed excellent photocatalytic activity for degradation of organic pollutants under visible-light irradiation.

The resultant products were firstly investigated by field emission scanning electron microscopy (FE-SEM). Figure 1(a) shows typical FE-SEM of Mn-doped BiOBr microspheres with ~1% Mn content. As-prepared products are found to contain intact microspheres as well as broken microspheres of ~1–3 µm in size. Higher magnification microscopy image [Fig. 1(b)] displays the surface information of the microspheres, which are constructed by numerous interlaced nanosheets with a thickness of about 20 nm. In addition, the feature of inner structure of the resultant microspheres is confirmed by Fig. 1(c), showing the compact central core with a sheet-like shell. The diameter of the core and thickness of the shell are about 500 and 750 nm, respectively. The cross-sectional image of the resultant microspheres is similar to the bloomed chrysanthemums, as shown in Fig. 1(d). The energy dispersive x-ray spectroscopy spectra (see Fig. S1 in ESI) reveal the presence of Mn, Bi, O, and Br elements and the content of Mn doping is about 1.05 wt%. The size and morphologies of the Mn-doped BiOBr microspheres depend on the content of doped Mn. At a lower content of doped Mn, the products still retain there flower-like morphology with ~1–3 µm in size similar to pure BiOBr. However, by increasing the amount of doped Mn in the composites, the uniformity of size and morphology decreases accordingly, as shown in Fig. S2.

Figure 1.

(a) SEM and its higher magnification (b) images of Mn-doped BiOBr microspheres with ~1% doped Mn. The cross-sectional image of the resultant microspheres (c) is similar to the bloomed chrysanthemums (d). TEM image of the resultant microspheres (e) and its HR-TEM image (f) and SAED pattern (g).

Figure 1(e) shows the transmission electron microscopy (TEM) image of the Mn-doped BiOBr microspheres. In agreement with the SEM findings, the roughness microspheres ~2 µm in size can be observed while the inner cores are clearly revealed by the sharp contrast between outer shells and solid interiors. Furthermore, the high-resolution TEM (HRTEM) image taken near the particle edges reveals distinct lattice fringes with a d spacing of 0.29 and 0.33 nm, which are slightly larger than the lattice plane distance of 0.277 nm for (110)^[Reference Song, Gao, Wang, Li, Liu, Xing and Zhang^28] and smaller than 0.30 nm for (011)^[Reference Cao, Xu, Lin, Luo and Chen^29] of un-doped BiOBr [Fig. 1(f)]. This can confirm that the doping of Mn affects the crystal structure of the BiOBr microspheres. The selected area electron diffraction (SAED) pattern of the Mn-doped BiOBr microspheres shown in Fig. 1(g) reveals that the sample has a polycrystalline nature.

The crystallographic structure of the Mn-doped BiOBr sample was further characterized by x-ray diffraction [XRD, Fig. 2(a)]. All of the diffraction peaks can be well indexed to the tetragonal phase BiOBr whose lattice parameters are a = 3.915, c = 8.076 Å (space group: P4/nmm, JCPDS 73-2061) before and after Mn doping. However, it is worth noting that the intensity of peak at 2θ = 11.5^o decreases by increasing the content of doped Mn and other characteristic peaks of Mn-doped BiOBr composites only have a slight shift compared with that of pure BiOBr, which may be attributed to the substitution of the Bi³⁺ ions (r _Bi = 1.17 Å) by the smaller radius Mn²⁺ ions (r _Mn = 0.86 Å).^[Reference Rao, Raju, Gopal and Narasimhulu^30] Furthermore, the porous structure of the Mn-doped BiOBr microspheres was also characterized by N₂ absorption analysis, which shows that the adsorbed volume of the Mn-doped BiOBr sample is comparatively higher. The Brunauer-Emmet-Teller (BET) specific surface area of the Mn-doped BiOBr sample was calculated to be 26.0 m²/g, which is much larger than the value (15.4 m²/g) of BiOBr due to the former hierarchical 3D nanostructured structure. The corresponding pore size distribution curves of the Mn-doped and pure BiOBr microspheres display a pore size distribution from 3 to 20 nm centered at ca. 14.5 and 20.5 nm, respectively, which confirmed that the as-prepared samples are mesoporous.

Figure 2.

XRD patterns of the Mn-doped BiOBr products (a) and N₂ absorption–desorption isotherm curves of the pure BiOBr microspheres (b) and Mn-doped BiOBr products (c).

To demonstrate the potential application of the as-prepared Mn-doped BiOBr microspheres, RhB, which is typically fluorescent dye, was employed as a model contamination to evaluate the photocatalytic activity of the samples. As shown in Fig. 3(a), RhB self-photo-degradation is almost negligible. As observed, about 78% of RhB is degraded under daylight lamp irradiation (11 W and 25 Hz) for 140 min using pure BiOBr as a photocatalyst. The Mn-doped BiOBr microspheres display highly efficient photocatalytic activity. About 88% of RhB is degraded using Mn-doped BiOBr microspheres with ~0.5% doped Mn as a photocatalyst under the same experimental conditions. Furthermore, by increasing the content of doped Mn in the photocatalysts, higher photocatalytic activity can be obtained. About 96.5 and 93.4% of RhB are degraded using Mn-doped BiOBr microspheres with ~1 and 5% doped Mn photocatalysts. However, excessive Mn content in the composites is not helpful in increasing their photocatalytic activity. Only 84.7% of RhB is degraded using Mn-doped BiOBr microspheres with ~10% doped Mn as a photocatalyst.

Figure 3.

Photocatalytic degradation of RhB using Mn-doped BiOBr photocatalysts (a) and trapping experiment of active species during the photocatalytic reaction (b).

For investigation of a plausible reaction mechanism for the superior photocatalytic activity of Mn-doped BiOBr microspheres and detecting the active species during photocatalytic reactivity, hydroxyl radicals (•OH), the superoxide radical (O₂^•−), and holes (h⁺) were investigated by adding 1.0 mM isopropyl alcohol (a quencher of •OH), p-benzoquinone (a quencher of O₂^•−), and triethanolamine (a quencher of h⁺), respectively.^[Reference Ye, Liu, Gong, Tian, Peng and Zan^31] The method was similar to the former photocatalytic activity test. Figure 3(b) displays the trapping experiment of active species during the photocatalytic reaction. It can be seen that the photocatalytic degradation of RhB is not affected by the addition of 1 mM isopropyl alcohol (IPA) (a quencher of •OH). On the contrary, the photocatalytic degradation of RhB obviously decreased with the addition of triethanolamine (TEOA) (a quencher of h⁺) and p-benzoquinone (BQ) (a quencher of O₂^•−). Therefore, it can be concluded that the holes and O₂^•− are the main active species of Mn-doped BiOBr microspheres in aqueous solution under visible light irradiation, rather than •OH. The possible reasons are given as follows: on the one hand, doping tends to change the lattice parameters. Owing to its relatively smaller ionic radius, Mn³⁺ ions may enter into the lattice of BiOBr. On the other hand, the doping of Mn to BiOBr led to reduction of the E _g of BiOBr by forming interband-gap localized levels^[Reference Abdollahi, Abdullah, Zainal and Yusof^32, Reference Liu, Zuo, Sun and Liang^33]. The charge-transfer transition may occur between the d-electron of dopant and the valence band or conduction band that can be induced by light excitation and lead to the effective separation of photogenerated electron–hole pairs. The photogenerated electrons react with molecular oxygen to produce superoxide radicals for the enhanced oxidation of organic pollutant.

In summary, the highly active visible-light induced Mn-doped BiOBr microspheres have been prepared by the solvothermal method. The photocatalytic activity of the Mn-doped BiOBr samples is highly dependent on the Mn content. The enhanced photocatalytic activity was attributed to the improvement of photoadsorption and charge separation. The holes and O₂^•− are the main active species of Mn-doped BiOBr microspheres in aqueous solution under visible light irradiation, rather than •OH. This may pave the way for practical applications of Mn-doped BiOBr photocatalyst under sunlight.